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Performance Evaluation of WiMAX/IEEE 802.16 OFDM Physical Layer - page 34 / 107

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OFDM Transmitter

Data in

Modulation (QPSK, M­QAM, etc)

IFFT

D/A

Baseband OFDM signal

OFDM Receiver

Data out

De­Modulation (QPSK, M­QAM, etc)

FFT

A/D

Baseband OFDM signal

Figure 3.3: Basic OFDM transmitter and receiver

At the receiver side, The FFT block performs the reverse process on the received signal and bring it back to frequency­domain. The block diagram in Figure de.picts the switch between frequency­domain and time domain in an OFDM system.

3.3.3 CYCLIC PREFIX ADDITION

The

subcarrier orthogonality of an OFDM system can be jeopardized when passes

through a multipath channel [23]. CP is used to combat ISI and ICI introduced by the multipath channel. CP is a copy of the last part of OFDM symbol which is appended to t h e f r o n t o f t r a n s m i t t e d O F D M s y m b o l [ 2 4 ] . T h e l e n g t h o f t h e C P ( T g ) m u s t b e c h o s e n a longer than the maximum delay spread of the target multipath environment. Fig 3.4 depicts the benefits arise from CP addition, certain position within the cyclic pre ix is chosen as the sampling starting point at the receiver, which satisfies the criteria s

m a x T x where max is the maximum multi­path spread. Once the above condition is satisfied, there is no ISI since the previous symbol will only have effect over samples within [, ax]. T g A n d i t i s a l s o c l e a r f r o m t h e f i g u r e t h a t s a m p l i n g p e r i o d s t a r t i n g f r o m T x w i l l e n c o m p the contribution from all the multi­path components so that all the samples experience the same channel and there is no ICI. a s s

23

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